1. Industrial Field of Utilization
The present invention relates to a method for restricting a region in magnetic resonance imaging, in order to selectively image a limited region inside an object.
2. Prior Art
The magnetic resonance imaging system (referred to as MR-CT hereinafter) for projecting a tomographic image which is obtained by utilizing the phenomenon of magnetic resonance (referred to as MR hereinafter) of a specific species of atomic nucleus as a target, has conventionally been known. The imaging based on Fourier transform technique will now be explained in detail hereinbelow.
The pulse sequence of one view, for applying a high-frequency magnetic field and gradient magnetic fields, is constructed as shown in FIG. 4. In the figure, RF represents a high-frequency, rotating magnetic field, imposing a 90.degree. pulse and a 180.degree. pulse to the x axis. G.sub.x is a fixed gradient magnetic field which is applied on the x axis called lead axis; G.sub.y is a variable gradient magnetic field applied to the y axis called warp axis; G.sub.z is a fixed gradient magnetic field which is applied on the z axis called slice axis. The signal represents spin-echo signal, i.e. SE signal, after a 180.degree. pulse. The intervals are shown in order to indicate when a signal from gradient magnetic fields is applied to each axis. In the interval 1, spins in the slice plane vertical to the z axis are selectively excited by a 90 pulse and a slice gradient, G.sub.z.sup.+. G.sub.x.sup.+ in the interval 2 is referred to as diffuse gradient, which disturbs and inverse a spin phase using a 180.degree. pulse. G.sub.z.sup.- functions for recovering the original spin phase after disturbance by G.sub.z.sup.+. In the interval 2, a warp gradient G.sub.yn is also applied. This functions for shifting the spin phase, in proportion with the position on the y direction. The intensity thereof is controlled so that it may vary in each interval. In the interval 3, a 180.degree. pulse is imposed to observe an SE signal appearing in the interval 4. In the interval 4, G.sub.x.sup.+ is a readout gradient, which is also a gradient magnetic field to align a disturbed magnetic field to induce an SE signal. The center of an SE signal is located at the position where the area of the readout gradient equals that of the diffuse gradient. Such sequence is repeated every pulse repeating interval T.sub.R, for a given number of views.
In MR-CT described above, there is obtained a tomographic image on a slice plane which is excited selectively along the z axis due to a 90.degree. pulse and a gradient magnetic field G.sub.z.sup.+ called slice gradient. According to the so-called three dimensional zoom scan comprising restricting the region of a tomographic image to the directions of x and y axes and selectively imaging the region 23 for zooming up, as is shown in FIG. 6, the region should be restricted on the directions of x, y and z axes. In the figure, 21 is a display such as CRT, which displays a tomographic image 22. 23 is a field of vision (referred to as FOV hereinafter), which is obtained through three dimensional zoom scan. By the three dimensional zoom scan, the region is restricted to the three directions comprising the thickness direction together with the directions of FOV 23. If imaging is carried out without restricting the region in such manner, there may occur an overlapping of an image within FOV 23 with folded images in the region exceeding FOV 23. For that reason, the selective excitation of magnetization vector has been carried out conventionally, by applying individual gradient fields together with a 90.degree. excitation pulse and a 180.degree. inversion pulse for restricting the region to two directions, a warp direction 1 and a warp direction 2. Because it has not been possible on a sequence to restrict the region to a readout direction as the remaining direction, however, an electric filter has been used to restrict the region by its band filtering, when an echo signal is received and detected. Because the positional difference in the intensity of a magnetic field is induced by the gradient magnetic field from the readout gradient, there may occur the positional difference in the Lamour frequency, which enables the region restriction by detection with an electric filter.
The sequence of the conventional three dimensional Fourier technique is shown in FIG. 2. In the FIG., 31 is a 90.degree. excitation pulse, which simultaneously applied to the z axis a slice gradient 32 corresponding to the pulse width of the 90.degree. excitation pulse. 33 is a warp gradient which is applied to the z axis; 34 is a warp gradient applied to the y axis, which imposes a slice gradient 36 on the y axis at the application of a 180.degree. pulse, and restrict the region along the two directions, the y and z axes. 37 is a diffuse gradient applied on the x axis, which produces an SE signal (figures are not shown) using the readout gradient 38. Spoilers 39a, 39b and 39c disturb magnetization vector after signal detection, to eliminate its influence over the following view. According to the conventional technique, the region restriction is carried out along the y and z axes, while the frequency component of the signals produced along the x axis is subjected to the restriction by an electric filter, to obtain a three dimensional image.
The technique carried out by restricting the region using an electric filter, requires the electric filter to have a very sharp cut-off property. It is very difficult to obtain a filter with such property in view of technology and even if it should be obtained, it might cost very expensive.